CONTACT ORGANS IN KILLIFISHES OF WOODS HOLE. 



177 



ever, that served to reveal the essential histological structure 

 almost as well as the usual method of sectioning and staining. 

 Scales or fins were soaked for a day or more in rather strong 

 KOH solution. This treatment renders all tissues except the 

 osseous parts quite transparent, so that, by carefully manipulat- 

 ing the illumination, practically all details were revealed under 

 low powers of the microscope. 



FIG. II. Camera drawing (X 60) of a single contact organ on a ray of the anal 

 fin of a male F. ma/a/is, cleared in KOH solution. The unshaded central portion 

 represents the osseous portion of fin ray and contact organ, showing the continuity 

 of the two. The stippled region is the dermis overlying the osseous portions. The 

 unshaded marginal portion represents the epidermis. 



On the scales (see Fig. 7) the spike-like core is continuous with 

 the newest layer of osseous material, while on the fins (see Fig. 

 1 1) the core is of a piece with the outer layer of the fin rays. In 

 other respects the organs are alike in both locations. Surround- 

 ing the core which is dense and hard, although somewhat elastic, 

 is a thick layer of dermis, more abundant near the tip of the or- 

 gan. The comparatively thin epidermis seemed to be stretched 

 over the tip of the organ and was frequently broken. Near the 

 base, however, it was as thick or thicker than the dermis of that 

 region. The difference between these structures and pearl or- 

 gans may be seen at once in the comparatively secondary impor- 

 tance of the epidermis and in the core of dermis with its osseous 

 support. The pearl organ is merely an epidermal callosity. 



